This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The reversible phosphorolysis of purine and pyrimidine bases is a very important biochemical reaction in the salvage pathway, which provides an alternative to the de novo biosynthetic pathways of purine and pyrimidine. There are two distinct folds catalyzing this reaction, nucleoside phosphorylase-I and II. The class I includes purine nucleoside phosphorylase (PNP) and uridine phosphorylse (UP), accepting purine nucleosides and uridine as substrates respectively. The previously reported PNPs are hexameric in prokaryotes and trimeric in higher eukaryotes like mammals. The reported prokaryotic UPs are hexameric and our study shows that in mammals they exist as dimers. Despite the difference in substrate specificity and quaternary structure, the members of this family share a common fold. The mammalian PNP and UPs are attractive targets for the design of chemotherapeutic drugs. Our current research includes structural characterization and mechanistic understanding of bovine UP. Structural studies are being carried out on Trichomonas vaginalis PNP (TvPNP) with different substrates that may provide insights into a new approach to chemotherapeutic control of this pathogen. An interesting on-going study in our laboratory, involves Streptococcus pyrogenes uridine phosphorylase (SpUP) that, based on kinetic studies, accepts both purine and pyrimidine as substrates. Another class of enzyme regulating the nucleoside pool within the cell is N-deoxyribosyltransferase, catalyzing the transfer of deoxyribosyl group from a purine/pyrimidine deoxynucleoside to a purine/pyrimidine base. Based on the substrate specificity this class is also divided into two subclasses, I and II. The type I accepts only purine, as both donor and acceptor base. The structure of purine deoxyribosyltransferase (PTD) has been published from Lactobacillus helveticus. The current focus is in solving the structure of LfPTD with N-2,3-ethenoguanine, which is a promutagenic and genotoxic DNA adduct. The structure of human deoxycytidine kinase (DCK) has been published with different nucleoside analogues. The study provides a structural basis for developing more effective drugs.